http://tes.nexusmods.com/downloads/file.php?id=14065

Would a levelling mod like this be possible in Skyrim? I remember that GCD added so much to my Morrowind, and nGCD to my Oblivion experience. They totally took away the constraints of standard levelling, and added so much to my immersion.

Any thoughts?

http://skyrim.nexusmods.com/downloads/file.php?id=3198

I remember the same but I don't think those mods are needed as much in Skyrim. Skyrim has better scaling and allows all skills to count towards leveling. It still retains the level-up menu but, for me, that's not enough reason by itself.
I guess I would consider using a comprehensive overhaul that also adjusted the scaling further like with OOO but it's hard to imagine one that would make everyone happy.

Not possible yet without Script Dragon or something similar.
I have no doubt SKSE will enable manipulation of the appropiate data at some point, but not yet.